Three esters of N-acyl phenyl isoserine, taxol, taxotere and cephalomannine have been found to possess significant properties as antitumor agents. This application describes a process for the preparation of N-acyl, N-sulfonyl and N-phosphoryl substituted isoserine esters, in general and to a semi-synthesis for the preparation of taxane derivatives such as taxol, taxotere and other biologically active derivatives involving the use of metal alkoxides and xcex2-lactams, in particular.
The taxane family of terpenes, of which taxol is a member, has attracted considerable interest in both the biological and chemical arts. Taxol is a promising cancer chemotherapeutic agent with a broad spectrum of antileukemic and tumor-inhibiting activity. Taxol has the following structure: 
wherein Ph is phenyl and Ac is acetyl. Because of this promising activity, taxol is currently undergoing clinical trials in both France and the United States.
The supply of taxol for these clinical trials is presently being provided by the bark from Taxus brevifollia (Western Yew). However, taxol is found only in minute quantities in the bark of these slow growing evergreens, causing considerable concern that the limited supply of taxol will not meet the demand. Consequently, chemists in recent years have expended their energies in trying to find a viable synthetic route for the preparation of taxol. So far, the results have not been entirely satisfactory.
One synthetic route that has been proposed is directed to the synthesis of the tetracyclic taxane nucleus from commodity chemicals. A synthesis of the taxol congener taxusin has been reported by Holton, et al. in JACS 110, 6558 (1988). Despite the progress made in this approach, the final total synthesis of taxol is, nevertheless, likely to be a multi-step, tedious, and costly process.
A semi-synthetic approach to the preparation of taxol has been described by Greene, et al. in JACS 110, 5917 (1988), and involves the use of a congener of taxol, 10-deacetyl baccatin III which has the structure of formula II shown below: 
10-deacetyl baccatin III is more readily available than taxol since it can be obtained from the needles of Taxus baccata. According to the method of Greene et al., 10-deacetyl baccatin III is converted to taxol by attachment of the C-10 acetyl group and by attachment of the C-13 xcex2-amido ester side chain through the esterification of the C-13 alcohol with a xcex2-amido carboxylic acid unit. Although this approach requires relatively few steps, the synthesis of the xcex2-amido carboxylic acid unit is a multi-step process which proceeds in low yield, and the coupling reaction is tedious and also proceeds in low yield. However, this coupling reaction is a key step which is required in every contemplated synthesis of taxol or biologically active derivative of taxol, since it has been shown by Wani, et al. in JACS 93, 2325 (1971) that the presence of the xcex2-amido ester side chain at C13 is required for anti-tumor activity.
More recently, it has been reported in Colin et al. U.S. Pat. No. 4,814,470 that taxol derivatives of the formula III below, have an activity significantly greater than that of taxol (I). 
Rxe2x80x2 represents hydrogen or acetyl and one of Rxe2x80x3 and Rxe2x80x3xe2x80x2 represents hydroxy and the other represents tert-butoxy-carbonylamino and their stereoisomeric forms, and mixtures thereof.
According to Colin et al., U.S. Pat. No. 4,418,470, the products of general formula (III) are obtained by the action of the sodium salt of tert-butyl N-chlorocarbamate on a product of general formula: 
in which Rxe2x80x2 denotes an acetyl or 2,2,2-trichloroethoxy-carbonyl radical, followed by the replacement of the 2,2,2-trichloroethloxycarbonyl group or groups by hydrogen. It is reported by Denis et al. in U.S. Pat. No. 4,924,011, however, that this process leads to a mixture of isomers which has to be separated and, as a result, not all tie baccatin III or 10-deactylbaccatin III employed for the preparation of the product of general formula (IV) can be converted to a product of general formula (III).
In an effort to improve upon the Colin et al. process, Denis et al. disclose a different process for preparing derivatives of baccatin III or of 10-deactyl-baccatin III of general formula: 
in which Rxe2x80x2 denotes hydrogen or acetyl wherein an acid of general formula: 
in which R1 is a hydroxy-protecting group, is condensed with a taxane derivative of general formula: 
in which R2 is an acetyl hydroxy-protecting group and R3 is a hydroxy-protecting group, and the protecting groups R1, R3 and, where appropriate, R2 are then replaced by hydrogen. However, this method employs relatively harsh conditions, proceeds with poor conversion, and provides less than optimal yields.
A major difficulty remaining in the synthesis of taxol and other potential anti-tumor agents is the lack of a readily available method for easy attachment, to the C-13 oxygen, of the chemical unit which provides the xcex2-amido ester side chain. Development of such a process for its attachment in high yield would facilitate the synthesis of taxol as well as related anti-tumor agents having a modified set of nuclear substituents or a modified C-13 side chain. This need has been fulfilled by the discovery of a new, efficient process for attachment, to the C-13 oxygen, of the chemical unit which provides the xcex2-amido ester side chain.
Another major difficulty encountered in the synthesis of taxol is that known processes for the attachment of the xcex2-amido ester side chain at C-13 are generally not sufficiently diastereoselective. Therefore the side chain precursor must be prepared in optically active form to obtain the desired diastereomer during attachment. The process of this invention, however, is highly diastereoselective, thus permitting the use of a racemic mixture of side chain precursor, eliminating the need for the expensive, time-consuming process of separating the precursor into its respective enantiomeric forms. The reaction additionally proceeds at a faster rate than previous processes, thus permitting the use of less side-chain precursor than has been required by such previous processes.
Among the objects of the present invention, therefore, is the provision of a process for the preparation of N-acyl, N-sulfonyl and N-phosphoryl esters of isoserine; the provision of a side chain precursor for the synthesis of taxane derivatives; the provision of a process for the attachment of the side chain precursor in relatively high yield to provide an intermediate which is readily converted to the desired taxane derivative; and the provision of such a process which is highly diastereo-selective.
In accordance with the present invention, a process is provided for the preparation of isoserine esters having the formula: 
comprising reacting a xcex2-lactam with a metal alkoxide, the xcex2-lactam having the formula: 
and the metal alkoxide having the formula:
MOCE1E2E3 
wherein
R1 is xe2x80x94OR6, xe2x80x94SR7, or xe2x80x94NR8R9;
R2 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;
R3 and R4 are independently hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, or acyl, provided, however, that R3 and R4 are not both acyl;
R5 is xe2x80x94COR10, xe2x80x94COOR10, xe2x80x94COSR10, xe2x80x94CONR8R10, xe2x80x94SO2R11, or xe2x80x94POR12R13,
R6 is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, hydroxy protecting group, or a functional group which increases the water solubility of the taxane derivative,
R7 is alkyl, alkenyl, alkynyl, aryl, heteroaryl, or sulflhydryl protecting group,
R8 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;
R9 is an amino protecting group;
R10 is alkyl, alkenyl, alkynyl, aryl, or heteroaryl,
R11 is alkyl, alkenyl, alkynyl, aryl, heteroaryl, xe2x80x94OR10, or xe2x80x94NR8R14,
R12 and R13 are independently alkyl, alkenyl, alkynyl, aryl, heteroaryl, xe2x80x94OR10, or xe2x80x94NR8R14,
R14 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;
E1, E2 and E3 are independently hydrogen, hydrocarbon or cyclic, provided, at least one of E1, E2 and E3 is other than hydrogen. Preferably, two of E1, E2, and E3 together with the carbon to which they are attached comprise a mono- or polycyclic skeleton.
In accordance with another aspect of the present invention, the metal alkoxide and xcex2-lactam are selected so as to provide a process for preparing taxol, taxotere and other biologically active taxane derivatives having the following structural formula: 
wherein
R1-R14 are as previously defined,
R15 and R16 are independently hydrogen, hydroxy, lower alkanoyloxy, alkenoyloxy, alkynoyloxy, aryloyloxy or R15 and R16 together form an oxo;
R17 and R18 are independently hydrogen or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy or R17 and R18 together form an oxo;
R19 and R20 are independently hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy;
R21 and R22 are independently hydrogen or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy or R21 and R22 together form an oxo;
R24 is hydrogen or hydroxy or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy; or
R23 and R24 together form an oxo or methylene or
R23 and R24 together with the carbon atom to which they are attached form an oxirane ring or
R23 and R22 together with the carbon atom to which they are attached form an oxetane ring;
R25 is hydrogen, hydroxy, or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy or
R26 is hydrogen, hydroxy, or lower alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy; or R26 and R25 taken together form an oxo; and
R27 is hydrogen, hydroxy or lower alkoxy, alkanoyloxy, alkenoyloxy, alkynoyloxy, or aryloyloxy.
Briefly, therefore, the taxane derivatives are prepared by reacting a xcex2-lactam (2) with a metal alkoxide having the bi-, tri- or tetracyclic taxane nucleus to form a xcex2-amido ester intermediate. The intermediate is then converted to the taxane derivative. xcex2-lactam (2) has the general formula: 
wherein R1-R5 are as previously defined. The metal alkoxide preferably has the tricyclic taxane nucleus corresponding to the general formula: 
wherein M is a metal, and R15-R27 are as previously defined. Most preferably, the metal alkoxide has the tetracyclic taxane nucleus corresponding to metal alkoxide (3) wherein R22 and R23 together form an oxetane ring.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.